1. Field of the Invention
It is known to make inorganic fibres from an inorganic melt using fiberising means comprising a set of rotors each mounted for rotation about a different substantially horizontal axis and arranged such that, when the rotors are rotating, melt poured onto the periphery of the top rotor in the set is thrown on to the periphery of the subsequent rotor (or onto the periphery of each subsequent rotor in sequence) in the set and inorganic fibres are thrown off the or each subsequent rotor.
This general process and apparatus can be used for different types of mineral melt. Success in the process depends critically upon the viscosity-temperature relationship and the surface tension of the melt at various stages in the process. Apparatus designed for a melt having one set of properties is wholly unsuitable for making fibres from a melt of a totally different properties.
2. Description of the Prior Art
In U.S. Pat. No. 4,238,213, Pallo describes the manufacture of ceramic fibres from such a fiberising means in which the set of rotors consists of two rotors that, in all the specific description, are described as having the same size and speed of rotation. In particular, the rotors have diameters of 150 to 300 mm and rotate at speeds of above 107 m/s and it is stated that these high speeds lead to fine average and effective fibre diameters than are obtained at lower speeds. However it is also stated that the process does lead to the presence of coarse shot in the fibre.
A particular problem with ceramic materials is that they do not melt until very high temperatures (typically around 1,800.degree. C.) but then change from a highly viscous state (at which they are too viscous to form fibres satisfactorily) to a highly fluid state (at which their viscosity is too low to form fibres) within a range typically of around 50.degree. C.
As a result, it is not practicable to operate such a process with more than two rotors since the melt on the third rotor would inevitably be too cool to form satisfactory fibres.
We are concerned with the production of mineral wool. Whereas aluminum silicate fibres consist of a minimum of 98% Al.sub.2 O.sub.3 and SiO.sub.2 and no more than 2% other oxides, the chemical composition of what we refer to herein as mineral wool is characterised by a large variety of oxides, where the sum of Al.sub.2 O.sub.3 and SiO.sub.2 is generally between 40 and 70%, and the rest is other common oxides from minerals, for instance CaO, MgO, Fe.sub.2 O, FeO, TiO.sub.2 or Na.sub.2 O, as described in U.S. Pat. No. 2,576,312 and in Danish Patent Application DK 4923/89. The raw material for this mineral wool, is normally composed of one or more of diabase, basalt, slag, lime-stone, dolomite, cement, clay, feldspart, sand or olivin or other relatively impure, usually iron-containing materials in which event the mineral wool is referred to herein as stone wool. Another type of mineral wool is glass wool, usually made from an iron-free melt containing expesive additives such as soda and borex. Both stone wool and glass wool can be produced at a much wider temperature range--about 200.degree. C.--typically 1,400 to 1,600.degree. C. Because of this fundamental difference in the melting and rheology properties and the demand for high output when making mineral wool, processes and apparatus suitable for ceramic fibres are not suitable for efficient and economic production of mineral wool.
Mineral wool, however, is made from rock (including slag) at lower temperatures and typically satisfactory formation of mineral wool fibres can be achieved through a range of as much as 200.degree. C. or more, typically 1,400 to 1,600.degree. C. Because of this fundamental difference in the melting and rheology properties, processes and apparatus suitable for ceramic fibres are not suitable for efficient production of mineral wool fibres.
Apparatus for making mineral wool fibres from a mineral melt of, for instance, slag or other stone comprises
a fiberising chamber, PA1 fiberising means in the chamber for receiving mineral melt, converting it into mineral wool fibres and air supply means for blowing the fibres axially along the chamber, and PA1 collector means comprising a conveyor in the base of the chamber for collecting the blown fibres as a web and for carrying them away from the fiberising means. PA1 characterised in that the top rotor is provided with driving means and has a size such that it can rotate to give an acceleration field of above 5O km/s.sup.2 and the second and third rotors are each provided with driving means and have a size such that each can give a greater acceleration field than the top rotor, and the axes of the first and second rotors are arranged such that a line drawn from the axis of the first rotor to the axis of the second rotor makes an angle of from 0 to 20.degree., preferably 5 to 10.degree., below the horizontal. PA1 where G=r.OMEGA..sup.2 PA1 where r is the radius of the rotor and PA1 .OMEGA. is the angular velocity of the rotor where ##EQU1## where n is the revolutions per minute.
There is an air supply for carrying fibres axially from the rotors. The air supply can be arranged merely around the periphery of the set (as in U.S. Pat. No. 3,709,670) or there can be an air supply slot associated with the or each of the said subsequent rotors. The slot can be spaced away from the periphery of the rotor as in EP 59152 or can be close to it, as in GB 1,559,117.
The literature proposes various diameters and speeds for the various rotors and a typical apparatus consists of four rotors with each rotor in the series being significantly larger than, and rotating at a higher peripheral velocity than, the preceding rotor. Typically the final rotor has a diameter almost or about twice the diameter of the top rotor and rotates with a peripheral velocity that typically is three times the peripheral velocity of the top rotor.
The acceleration field that can be imparted by the final rotor is very much more than (for instance five times) the acceleration field that can be imparted by the first rotor but it is not commercially practicable to try to achieve finer diameters by increasing the speed still further because of the extreme engineering and material problems in arranging for the final rotor or rotors to travel at faster speeds. For instance increasing the peripheral velocity of a final rotor of 33O mm diameter above its typical present maximum of around 7,000 rpm leads to severe risk of shattering of the rotor, unless the rotor is made of exceedingly expensive material. It is proposed in WO/90/15032 to mount a rotor having a diameter of 300 to 400 mm on magnetic bearings so as to permit high speeds of revolution, but this still creates engineering and economic difficulties.
There are numerous disclosures in the literature of fiberising means for mineral wool comprising three or four fiberising rotors arranged in the general manner described above, and the top rotor in the set always has a smaller diameter and/or a lower peripheral velocity than all the other rotors, with the result that the acceleration force that it can impart is very much less than the acceleration force provided by each of the other rotors. For instance a typical construction of the type shown in GB 1,559,117 gives an acceleration force on the top rotor of around 15 km/s.sup.2 (15000 metres per second.sup.2) whereas the acceleration force provided by each of the other rotors ranges from 2 to 5 times as much.
The principle reason for this is that conventional thinking has dictated that it would be impossible to accelerate the melt by the top rotor alone sufficient to give the capacity for forming fibres on that rotor and that it should, instead, merely serve to accelerate the melt sufficiently for it to be thrown against the second rotor with sufficient force that fibres will then satisfactorily be formed on the second rotor.
Conventional thinking has also indicated that each rotor in the set should have a larger diameter and a higher peripheral velocity than each preceding rotor in the set, with the result that the most effective fibre formation is achieved off the last rotor, where the acceleration force is greatest, typically in the range 50 to 100 km/s.sup.2 compared to a value of around 15 km/s.sup.2 off the first rotor.
U.S. Pat. No. 2,520,168 discloses fiberising apparatus comprising rotors each of which is supported by a shaft that makes an angle to the horizontal. This document also discloses that the top rotor is driven at a slower speed than the lower rotors, as is discussed above.
Typically no particular consideration has been applied to the vertical and horizontal displacement of the rotors with respect to each other, and in particular they have been arranged relatively close to one another with their particular positions selected more for engineering convenience than for optimisation of fibre formation.
Whenever mineral wool fibres are made using such fiberising apparatus, a problem that arises is that some of the melt is thrown off the fiberising apparatus in the form of shot, that is to say coarse particles of melt. If this shot is trapped in the mineral wool, it reduces the properties of the wool. If, alternatively, it is collected from the base of the chamber it has to be recycled and this reduces the overall efficiency of the process. It would be desirable to reduce the formation of shot during the manufacture of mineral wool by a fiberising apparatus that consists of at least three of the described rotors.